It looks like ADO, which is a first class bus line (bathrooms and TV on board), ADO GL (All First Class benefits plus a snack and drinks as well as bigger seats, more pitch) and UNO (sleeper bus with HUGE seats and sandwiches, etc) run from the Cancun bus station to Merida bus station for anywhere between 22-36 dollars with a travel time of +/- 4.5 hours.